1. Field of the Invention
The present invention relates to the field of electronic circuits and in particular to a circuit for converting a differential signal into a nondifferential signal.
2. Description of the Related Art
In many electronic applications, and in particular in communication systems, signal is processed in a differential form in order to ensure optimal linearity and especially immunity to external disturbances.
That is particularly the case in the field of transmissions. If one considers, for example, a radio frequency (RF) transmission chain as illustrated in FIG. 1, it can be seen that an Intermediary Frequency (IF) signal 1 is mixed with a signal 2 generated by a local oscillator (LO) by means of a mixer 3 in order to produce an RF signal, which is then transmitted to a pre-power amplifier PPA 11. The immunity of the signal to external disturbances and the processing linearity of such a chain—illustrated only as an example here—are ensured by the differential mode used for signals and, in general, mixer 3 and pre-power amplifier 11 can be provided by means of a single integrated circuit. The output signal of amplifier 11 is transmitted to an impedance adaptor circuit 12, then to a circuit 15 known as a BALUN circuit that converts the signal into a nondifferential mode before transmission to a surface acoustic wave (SAW) filter 13, which filter is very selective, so that only the RF part of the signal is selected. After filtering, the nondifferential signal is transmitted to a power amplifier 14 for amplification before transmission to a transmitting antenna (not shown). The SAW filter and the PPA are generally not differential devices. This is why the BALUN circuit is used to convert the differential signal into a nondifferential signal.
As can be seen in the diagram of FIG. 1, BALUN block 15 carries out the conversion of a differential signal into a nondifferential signal. The disadvantage of such a structure lies in the fact that BALUN circuit 15 is external to the integrated circuit realizing the mixer and the pre-power amplifier, which significantly increases the function's manufacturing costs.
A second technique exists. It consists in using only one of the two differential outputs of the pre-power amplifier and replacing the differential adaptor circuit 12 by a nondifferential adaptor circuit 22. This technique makes it possible to avoid using the BALUN circuit. However, this solution is very power consuming and is not optimal as regards signal linearity.
A third known technique making it possible to remove the BALUN circuit consists in using a circuit of the CAPRIO-QUAD-type (an evolution of the Cross-Quad proposed by Caprio). This technique makes it possible to convert a differential signal into a nondifferential signal, but at the expense of power-consumption and linearity.
A fourth technique, known as “current combiner”, makes it possible to convert a differential signal into a nondifferential signal. This device comprises a tuned circuit based on a passive network of inductive resistors and capacitors. One disadvantage of this solution, amongst others, lies in the fact that inductors—when they are integrated into an integrated circuit—occupy considerable space on the silicon substrate and therefore increase the manufacturing cost of such a circuit. Moreover, the performances of this type of circuit largely depend on the precision with which its elements are manufactured.
Thus, it can be noted that a simple solution is yet to be found to allow integration of a circuit for converting a differential signal into a nondifferential signal, particularly adapted to the realization of an RF signal transmission chain, in a semiconductor substrate. This is why, in practice, many concrete realizations include a BALUN circuit outside the integrated circuit, the use of which BALUN circuit has an impact on emitter manufacturing costs.
It would be desirable to have a circuit occupying minimum space on a substrate and making it possible to convert a differential signal into a nondifferential signal.